TXE (Telephone eXchange Electronic) was a family of telephone exchanges developed by the British General Post Office (GPO), designed to replace the ageing Strowger switches.
When World War II ended, the UK telephone exchange suppliers supported the GPO's decision to stay with Strowger until a viable electronic system became available. The GPO largely did this to protect their success in the export market, but it actually had the effect of ultimately destroying it. This allowed competitors to develop their own improved switching systems ahead of the GPO. In 1960 the situation rapidly changed when the Australian Postmaster-General's Department rejected a system from a consortium of British manufacturers who offered a register-controlled version of a motor-uniselector system in favour of a crossbar system from LM Ericsson. Suddenly the rules had changed and the race was on to develop an electronic telephone exchange that could operate with the current GPO telephones used in the UK, including shared service.
Introduction Just before World War II Tommy Flowers, employed at the GPO, had been working on VF (voice frequency) signalling, using valves (vacuum tubes), and this had led him to realise that valves could be very reliable if not switched on and off. This gave him the confidence during the war to build the world's first digital computer, called Colossus, at Bletchley Park. After the war, the success of Colossus encouraged him to contemplate the possibility of telephone exchanges each using tens of thousands of valves. He was told that this was impossible and he could not say he had already done it with Colossus because he was bound by the Official Secrets Act. However, a fully electronic prototype Time-Division Multiplex Model Exchange was constructed at the Post Office Research Station at Dollis Hill and then an experimental TDM exchange system was built and tested at Highgate Wood in 1962, but it was found to be beyond the technology of the time: the solid-state switching worked well, but the analogue transmission (which had worked on the short cable runs of a laboratory model at Dollis Hill) was too noisy for public service on the long cable runs of a large exchange. However, the principles would be used later, as transmission became digital, in the development of digital exchanges the world over, including System X.
Siemens Brothers (later taken over by Associated Electrical Industries, who renamed each section accordingly, e.g. AEI Telecoms) had set up an electronic switching lab at Blackheath. This lab was headed by John Flood, who had been a founder member of Tommy Flowers' electronic switching team at Dollis Hill. The Siemens team included an engineer called Jim Warman, whose trunking ideas (sectionalisation, serial trunking, line scanning, route choice, repeat attempt etc.), which were to be central to the development of the British TXE exchanges.
Following the failure to win major contracts in Australia in 1960 and the subsequent failure of Highgate Wood, it was necessary for the British manufacturers to come up with something different until a fully digital system could be developed (which eventually turned out to be System X and System Y). Ericsson had twenty years of experience of manufacturing the crossbar system and reducing its cost, so there was no point in trying to compete with them (Plessey Telecommunications, a subsidiary of Plessey, took a different view and continued to urge the GPO to adopt crossbar). At this time, in the US, Bell Labs were developing a system based on electronically controlled reed relays, and this looked promising. One of Ericsson's marketing points for crossbar was that it used precious-metal contacts, but reed relays would be even better as their precious metal contacts were hermetically sealed. Also their very short operating and release times (<1 ms) made them ideal for electronic control, and these reed-electronic exchanges were considered the most practical switching system to proceed with at the time, and electronic enough until a truly electronic system could be developed; Tommy Flowers did not approve as he advocated going straight to a digital system.
The manager at AEI (W G Patterson) decided that reed-electronic space-division switching was the way to go, and it was then that the term 'TXE' (Telephone eXchange Electronic) was coined, even although the reed relays themselves were not regarded as electronic components.
A much bigger team was needed to undertake the detailed development and AEI persuaded AT&E and STC to join them in the work. The initial result of their work was a prototype system called TXE1.
TXE1
The TXE1 was developed by three members of the Joint Electronic Research Committee (JERC) which was formed in 1956 and lasted until 1969. The JERC consisted of the GPO, Siemens Brothers (later AEI), Automatic Telephone & Electric (later Plessey), the Ericsson Telephones (also later Plessey), General Electric Company (GEC), and Standard Telephones and Cables (STC). STC built the common control; AEI the switching and scanners, line scanning, and test console; and AT&E the dialling capturing equipment (registers) and the incoming and outgoing junctions. (A 'junction' in British telephonic terms was not a junction in the everyday sense, but the name given to the pair of wires connecting a call between a satellite exchange and the main exchange.) The development of the TXE1 started around 1963. There were models of the AEI equipment at Blackheath and the ATE equipment at Edge Lane, Liverpool. AEI called TXE1 their REX (reed electronic exchanges).
Completion was delayed but the TXE1 went into service in 1968 in Leighton Buzzard. Although designed to handle 10,000 subscribers, it started out with a capacity of 3000, with 152 incoming junctions and 166 outgoing junctions. Later, instead of expansion of the TXE1, capacity was increased with three TXE2 exchanges and a TXE6.
The exchange was housed in a prototype K-type single-story building on the site of the former Lake House in Lake Street. The construction included thermal insulation panels, double-glazing, and under-floor electrical heating. Ventilation arrangements were by eight ventilating units, each handling 600 cu. ft. per min, and a series of "hit-and-miss"-type louvers above the windows on each side of the building provided outlets for heated air.
It was withdrawn from service in 1977 when it was replaced by a TXE4.
Description of mechanism
An equipment practice was needed quickly, and it was realised that a matrix of reed relays would be about the same size as a crossbar switch. Therefore, the equipment practice of AT&E's crossbar system was adopted for the TXE1 apart from the common control, which had its own equipment practice. The common control consisted of 14 racks and made up a complete suite of the exchange. It was made entirely from discrete components as integrated circuits were not yet in common use. There was much discussion by all contractors as to whether there was a reliable connector available to make units interchangeable. STC decided to have the units that could be withdrawn and AT&E and AEI did not. It turned out that the connectors used were reliable and they had a great advantage in fault finding. It also allowed the STC engineers to place a suspect faulty unit in an outrigger so it could be tested in situ.
One of the functions of the common control was to decide which was the best connection to be used through the switching network, and this part was called the route choice. The interrogators would return the available paths and the route choice would make a choice and tell the markers to mark that route.
The exchange used reed relays as the switching medium, and the reeds themselves were approximately 3 inches in length and the only ones available. They were provided by the Hivac subsidiary of AT&E (then the only UK manufacturer of reed inserts.) It had multi-stage switching divided into A, B, and C switches, interconnected by links. A typical local call would be connected via A-B-C-Link-C-B-A. Links could each either have or not have transmission bridges for local calls. The bridges were contained within the outgoing junction units.
The exchange had some advanced features for the time, including multi-frequency (MF) tone dialling as an option opposed to pulse dialling, and no post-dialling delay for own-exchange calls. It also had the ability to detect a switching failure and automatically attempt a repeat dial. Any repeat attempts were logged to the teleprinter. It also had a test console, which monitored all calls on a digital edge-lit display. Another display gave a visual indication of the traffic flowing through the exchange, named the Hubblemeter after the instigator Ray Hubble. Occasionally the call trace did not work, but the engineers worked out a way of manually tracing a call. What they did was to buy a little compass and glue a piece of magnetic ferrite on the side to pull the compass needle away from north. They would then run this compass along outside of the reed relays, and when a relay was operated the needle would move back to north. This was repeated over several sets of the switching path until the trace was complete.
Inter-rack cabling was via a cable loft. The cables were routed up through packed pass-throughs into a reinforced ceiling.
A novel but subsequently disastrous feature designed by Bell Antwerp was used to hold the subscriber's class of service information i.e. PBX, shared service, incoming calls barred (ICB), temporary out of service (TOS), etc. This was a capacitor store, and it held information on a thin plastic strip, into which could be inserted up to 10 little copper squares which had a capacitance of 10 pF. The thin plastic strips were then inserted into the Data Store rack, one at the position representing the directory number and another at the position representing the equipment number. This can be seen in the photograph together with some plastic strips hanging by wire. Hanging the strips by wire was a common practice for subscribers who were constantly changing their Class of Service, i.e. being made TOS. This information was then pulsed by the common control translator and appropriate action taken. In the end the problem turned out to be cable interference requiring substantial re-cabling at the rear of the rack. This system was replaced in later TXE exchanges by Dimond rings.
The registers looked after all the dialling, and there were three sorts of registers: loop-disconnect, MF (later called DTMF), and incoming. There were about 20 local registers and 12 incoming registers. The local registers (loop-disconnect and MF) took care of own-exchange and outgoing calls, while the incoming registers dealt with calls coming into the exchange. A local register would provide dial tone to the subscriber, wait for the first dialled digit, and then apply to the translator to see what action was required. The translator could decide by the first digit if it was a local call, and if it was it would instruct the register to come back when it had all the digits. If it was not a local call and therefore to be routed out of the exchange, then it would tell the register to come back with each digit until it could decide on the routing, as not all calls went to the GSC (Group Switching Centre) as there was AAR (alternative available routing). Once the routing had been decided and digits passed on, the register was free to take another call.
The MF senders/receivers were used when an MF subscriber initiated a call. They were set up to the subscribers line and switching network to an MF register, they converted the MF tones to pulses for the registers to store. They used the X, Y, and auxiliary switching planes.